Small craters of a few millimetres diameter can be caused by stones hitting safety-glass windshields of automotive vehicles. Normally a cone-like crack or pockmark is formed thereby which extends inwardly to the plastics sheet provided between the laminated sheets of glass.
It is known to repair such damages by injecting a liquid adhesive. An especially suitable adhesive is constituted by a synthetic resin which is cured by exposure to light and which, in its cured state, is clear and has a refractive index similar to that of the glass to be repaired. The adhesive has two functions, viz. firstly, to restore the mechanical stability of the glass and, secondly, to eliminate undesirable optical refraction phenomena by filling any air gaps between the faces of the crack.
U.S. Pat. No. 3,765,975 discloses a device for performing such repairs, in which a block is placed on the windshield with a cavity surrounding the damaged portion and sealed to the windshield surface. The liquid resin is filled into said cavity, whereafter vacuum and pressure are alternatingly produced by means of a syringe in order to evacuate the mentioned air gaps and to inject the resin into the same. The cavity formed within the block is sealed relative to the windshield by means of an O-ring. In order to keep the device in place it is necessary to apply pressure to the block and thus to the windshield in the vicinity of the pockmark, thereby incurring the risk of enlarging the damaged area due to such pressure and making a repair finally impossible. The same risk will be incurred when, due to repeated pumping operations by means of the syringe, excessive pressure is inadvertently built up in the vicinity of the pockmark. Also, there is the risk of adhesive being injected between the plastics and glass sheets. It is a further drawback that more synthetic resin is filled into the said cavity than is necessary for filling the cracks in the windshield so that, after the completed repair, the amount of injected resin may cause a permanent stress within the crack which may in turn lead to later damage.
DE-C-No. 3,607,738 discloses another device in which vacuum is produced by a flexible suction cup placed over the pockmark and liquid resin is supplied to the pockmark by means of a syringe. The suction cup, which is made from transparent material, is penetrated by the needle of the syringe. In order to enable the suction cup to produce the vacuum, it must initially be placed with a corresponding pressure onto the windshield in the vicinity of the pockmark. Again there is a risk that because of such manually caused pressure the already sensitive area in the vicinity of the pockmark is subjected to excessive stress and the crack is thereby enlarged. It is another difficulty that the suction cup, even though it may be made from transparent material, makes it difficult to view the damaged area and accurately to inject the resin in the area at the apex of the cone-like pockmark. Also, penetration of the suction cup by the syringe requires some skill to ensure that the needle tip will accurately meet said cone apex. If this cannot be achieved, the user will try to vary the direction of the needle by moving the syringe, which may result in leakages at the suction cup and a corresponding loss of vacuum. Finally, there is the risk that upon penetration of the suction cup the syringe may be moved too far inwardly and cause further damage to the already damaged portion.
Furthermore, a device sold by Clear Star Products, Inc., is available on the U.S. market, in which a sandwich-type annular adhesive tape with a hole leaving the pockmark exposed is placed on the windshield. A base is affixed to the adhesive tape, said base being provided with an outwardly extending tubular projection terminating at the hole in the adhesive tape. Liquid synthetic resin is filled into said projection by means of a pipette. A syringe is then introduced, which is provided in its sidewall near the bottom end with a vent opening adapted to be closed by a pin. When the pin is inserted, a plunger is initially retracted so as to produce a vacuum at the pockmark and to remove air entrapped therein. In the retracted position, the plunger is retained by the locking action of a spring clasp. The pin is then withdrawn to release the vacuum and is subsequently re-inserted. Then the plunger is depressed to inject the resin under pressure into the pockmark. In the pushed-in pressure position, the plunger is retained by a further locking means. The alternating vacuum/pressure pumping operation can be repeated several times.
A similar manipulation is provided in a further device known from U.S. Pat. No. 4,419,305 which comprises a syringe with a discharge opening and a vent hole penetrating a wall of the syringe casing near the front end thereof, a plunger movable within the casing, and a seal for hermetically sealing the discharge opening and the damaged portion relative to the ambient atmosphere. This device is first placed on the windshield without the plunger inserted and in such a manner that the seal surrounds the damaged portion, whereupon a synthetic resin in filled into the syringe casing. The plunger is then inserted to a position behind the vent hole. In this condition vacuum is applied via a hose to a chamber which surrounds the syringe to remove any air trapped in the resin. The plunger is then forwarded by manual rotation applied to a handle provided at the rear end thereof to urge the de-gassed resin into the pockmark.
In the use of the latter two devices, the resin will at once penetrate into the pockmark due to capillary forces. These capillary forces counteract the applied vacuum and render it difficult to de-gas the resin. There also is a risk of excessive pressure being exerted by the manual pumping operation which may widen the crack. Further, the applied pressure may not only move the plunger but inadvertently press also the whole syringe against the windshield, so that the pockmark is enlarged by this mechanical action. Finally, a predetermined sequence of handling steps must be observed in the practical application of these devices, and the various component parts of the respective overall device must be assembled and operated as prescribed so that, in addition to requiring manual skill, operating instructions must be closely followed for the repair to be successful.